In a few places we need a unique ID to represent that object and use
that to compute hash key. Right now we are using Serial for that purpose
but it creates confusion with QueueSerial which we are using Serial to
track GPU progress. This CL changes these usage of Serial to
TextureSerial, SamplerSerial, ImageViewSerial type so that compiler can
perform type checking. It also adds BufferSerial in preparation for next
CL.

Bug: b/159457348
Change-Id: I8e2da69c2029e4ddbcf163981ae46f85e19f751b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2287426
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>